Image Processing
IAPY masterclass: Man Under the Stars.
How to blend land and sky, and retain the anchor of a human at the heart of it all
It is difficult to truly capture the awesomeness of our place in the Universe alongside the grandeur of the Scottish mountains in a single photograph. To capture the moment, you have to take multiple images and bring them together. Without any planning, a random stranger unknowingly added to the incredible sense of scale being striven for in this photograph. He stood on the moor, oblivious to my presence and dead centre to the mountain, Buachaille Etive Mor in the Scottish Highlands. The man is dwarfed by the mountain, and in turn the mountain is dwarfed by the Milky Way.
On location, the field of view that my camera could capture felt very limited. I wanted viewers to feel they were looking at the entirety of space, not just focusing on one part of the sky; stitching multiple photographs together was the answer. Using a Nikon 14-24mm lens on a Nikon D4S, a professional 16.2-million-pixel DSLR, gives a 114º viewing angle. Shooting
in portrait orientation, I used a geared tripod head to move the camera by 15º between frames. The overlap seemed like overkill, but the extra information came in handy later when stitching the images.
Each exposure was 20 seconds at f/2.8 and an ISO of 8000. I left the automatic dark frame reduction on as I have found it produces much cleaner images than my attempts at subtracting dark frames in Photoshop. It is also highly possible that the –6ºC night-time air helped the camera produce less noise. Overall it took about 14 minutes to capture the entire panorama. Any longer and star trailing might have started to cause problems.
Assembling the images without
artefacts was nerve-wracking. Lightroom and Photoshop both offer stitching options with limited control, but I used PTGui (www.ptgui.com; free trial), a panoramic image stitching software for Windows and macOS. PTGui will try to merge the images for you (press Load Images in the initial window, then Align Images), but it will usually fail. After this you are invited to set ‘control points’. This means you tell
the program which parts of the image are the same by matching up stars from different photographs. I’d recommend three to five per image.
Once I’d merged the panorama, I created a virtual copy of the photograph in Lightroom
(Photo > Create Virtual Copy), so I had two: one to show the ground and the other to show the stars. Starting with the ‘ground’ image, I used the white balance and colour hue sliders to ensure the snow was pure white. Then I found the exposure’s sweet spot so the snow looked bright and attractive, yet not glowing like in daylight.
Hunt for the hiker
At this point I was happy with how the image looked so loaded it into Photoshop. I went through the originals to find the image where the man had managed to stand still for 20 seconds (this is where having too much information helped as he appeared in seven of the frames), then cut and pasted him over the blurred image of him that appeared in the panorama.
I used Lightroom’s brush tool to boost the saturation and sharpening around him to make him stand out as much as possible. I adjust the size of the brush a lot depending on the area of the photo I am working on.
Now I could move on to the ‘stars’ image. I always want my images to look natural
and realistic so I try to be light-handed when editing. I used the white balance slider to add a slight blue tint to the sky to create the feeling of cold (because it was really, really cold) and adjusted the contrast and shadow sliders to bring out the stars.
I’m not a huge fan of emphasising the winter Milky Way but in this image, it needed to be done. Importing this photo into Photoshop I painted on a high pass sharpening filter (Filter >
Other > High Pass). I normally select a sharpening radius of around 150. Once this is applied it will instantly make the photo look horrible.
I fixed this by setting the blending mode to Soft Light (Layers > Blending Options > Soft Light) and hiding the layer mask (Layers > Layer Mask > Hide All). I used
the brush tool with 50 per cent opacity to subtly bring out the detail of the Milky Way.
At this point it was time to superimpose one photograph onto the other, achieved by clicking Select > All and then copying everything, switching to the other image and pasting. I then placed the layer that showed detail on the ground above all others, and created a layer mask over it
(Layers > Layer Mask > Hide All). After setting the blend mode to Normal, I used a white brush (opacity 100 per cent) to paint in the details of the ground I wanted to keep. The result was the image I entered into Astronomy Photographer of the Year 2016.